1. Field of the Invention
The present invention relates to a signal generating apparatus for generating various types of signals in a color television receiver, and more particularly to a signal generating apparatus for generating a convergence correction signal and an on-screen display (referred to as, "OSD" hereinafter) character signal simultaneously.
2. Description of the Prior Art
Generally in a video projector, three projection cathode-ray tubes are used for applying light beams of three primary colors of red (R), green (G) and blue (B) for expanding and projecting a video image picture onto a screen, where the incident projection angles (referred to as, "convergence angle" hereinafter) of the light beams from the projection cathode-ray tubes with respect to the screen are different from each other, resulting in that color deviation, focusing deviation, deflection distortion, and luminance change may occur in the picture displayed on the screen. Various corrections to such deviations have been implemented by a system in which analog-like correction waveforms are created in synchronization with horizontal and vertical scanning periods and the amplitude (size) and shape of the analog-like correction waveforms are varied to be adjusted for the corrections. However, the conventional system as mentioned above has been accompanied by a problem in correction accuracy.
Under such circumstances, there have been disclosed some digital convergence apparatuses in the U.S. Pat. Nos. 5,111,284 and 4,672,275 and in the Japanese Patent Unexamined Laid-Open 8114/1984, teaching a method of convergence correction with high accuracy. Besides, disclosed is a digital convergence apparatus in the U.S. Pat. 4,999,703 and the Japanese Patent Publication 38797/1991, teaching a method in which the correction is performed automatically. Further, disclosed is a digital convergence apparatus in the Japanese Patent Publication 5314/1992 capable of treating various scanning frequencies. Moreover, a multi-scan character display system is suggested in the Japanese Patent Unexamined Laid-Open 202784/1990 teaching a method of treating various scanning frequencies.
The signal Generating apparatus in the above-mentioned conventional video projector is described below with reference to FIG. 14.
Referring to FIG. 14, reference numeral 80 denotes a projection cathode-ray tube; 86 a screen; 87 a projection lens; 90 a convergence yoke; and 91 a deflection yoke. A video signal derived through an input terminal 60 is amplified to a necessary amplitude by a video circuit 81 to drive the projection cathode-ray tube 80. The video circuit 81, which performs the same operation as in the conventional receiver during a normal display mode, projects a crosshatch signal of a digital convergence circuit 82 as shown in the display screen in FIG. 15 during convergence adjustment. In this digital convergence correction, correction data of adjustment points displayed on the screen are stored and interpolation of the correction data among the adjustment points is performed to create a correction waveform. Accordingly, correction can be effected for each adjustment point independently, so that the correction can be implemented with high accuracy.
An OSD (on-screen display) character generator 88 generates OSD characters to be displayed on the display screen in accordance with a sync signal derived through an input terminal 70. An OSD character signal derived from the OSD character generator 88 is fed to the video circuit 81, where the video signal derived through the input terminal 60 is added to the character signal, so that characters are displayed on the display screen. A deflection circuit 83 and the deflection yoke 91 perform the scanning operation of electron beams in the projection cathode-ray tube 80 in synchronization with the sync signal derived through the input terminal 70. Although the projection cathode-ray tube 80 is shown only one in number in FIG. 14, three projection cathode-ray tubes for primary three colors of red (R), green (G), and blue (B) are normally used.
An adjustment pattern detector 85, which is made up of such as a camera for performing photodetection, detects a crosshatch pattern projected on the screen 86 and feeds a detection signal to the adjustment-point detector 84. The adjustment-point detector 84 detects convergence deviation of each adjustment point according to the detection signal fed from the adjustment pattern detector 85 and changes the amount of correction of the digital convergence circuit 82 according to the detected convergence deviation, thus automatically accomplishing convergence adjustment.
As described above, by detecting the convergence deviation of the display on the screen with a photodetector such as a camera and using the resulting detection signal to control correction data, there can be realized an automatically adjustable signal generating apparatus for generating a convergence correction signal and an OSD character signal simultaneously with high accuracy.
However, the signal generation apparatus of the above-described conventional arrangement has been accompanied by problems such that the convergence correction signal could not be generated at arbitrary scanning frequencies, and that the size of OSD characters is varied in accordance with the number of scanning lines. Moreover, in order to treat signal sources having different numbers of input scanning lines, there would arise the need of separately effecting interpolation of correction data among adjustment points and interpolation of character signals in the vertical direction, with the result of quite a large scale of circuitry.
The present invention has been developed with a view of solving the foregoing problems.